Leonardo engaged in the dissection of human bodies with particular intensity in the years around 1505. This pursuit is documented in many surviving drawings, rightly admired for their expressive power. Leonardo sought to demonstrate the close analogy between the machine and the human body. He saw both as the wonderful achievements of Nature, whose iron laws govern not only mechanical instruments but also the motions of animals. "Nature," Leonardo notes, "cannot impart motion to animals without mechanical instruments."
Not surprisingly, therefore, his anatomical investigations concentrate on the basic organs of the human body of which his drawings repeatedly underscore the direct analogy with mechanical devices - to the point of suggesting the possibility of building fully operational artificial limbs and models.
The reduction of the motions of the living organism to the elementary laws of mechanics is reflected in Leonardo's plan to use his book of the "elements of machines" as an introduction to his treatise on anatomy: "Put the book of the elements of machines before the demonstration of the motion and strength of man and other animals, and thanks to [those elements] you will be able to prove all your propositions."